GlobalISel: Use the correct types when translating landingpad instructions

There was a bug here where we were using p0 instead of s32 for the
selector type in the landingpad. Instead of hardcoding these types we
should get the types from the landingpad instruction directly.

Note that we replicate an assert from SDAG here to only support
two-valued landingpads.

llvm-svn: 292995